Method and means for floating-zone melting of rod-shaped bodies of crystallizable semiconducting or conducting material



April 20, 1965 T. RUMMEL 3,179,502 METHOD AND MEANS FOR FLOATING-ZONE MELTING OF ROD-SHAPED BODIES OF CRYSTALLIZABLE SEMICONDUCTING 0R CONDUCTING MATERIAL Filed March 16, 1962 Fig.1

Fig.4

United States Patent Ofiice 3,1795% Patented Apr. 20, 1965 METHGD AND MEANS 130R FLGATHNG-ZQNE MELHNG 01F RQD-SHAHED BQDKES F CRYS- TALLEZABLE SEMIQQENDUCTWG QR CON- DUCTENG MATERIAL Theodor Rurnrnel, Munich, Germany, assignor to Siemans & Halsire Ahtiengeseilschaft, Berlin, Germany, a corporation of Germany Filed Mar. 16, 1962, Ser. No. 180,216

Claims priority, application Germany, Mar. 17, 1%

11 claim. a. zs-sar My invention relates to method and means for the crucible-free zone melting of rod-shaped bodies consisting of crystallizable semiconducting or conducting material.

According to the method described in the copending application of K. Siebertz, Serial No. 409,420, filed February it), 1954, now Patent No. 3,086,856, assigned to the assignee of the present invention, such zone melting operation is performed by mounting the rods in vertical position, maintaining the zone molten by electric induction and simultaneously supporting the molten zone by levitating forces produced by an auxiliary induction coil concentrically arranged beneath the zone and traversed by alternating current. Described in Patent 2,686,- 864 is another floating-zone melting method which produces the molten zone in the vertically held rod by means of two induction coils vertically spaced from each other and simultaneously providing levitating action for the zone, the molten zone remaining located between the two coils While travelling longitudinally through the rod.

It has been found difficult to satisfactorily control the heating and levitation of the molten zone when employtwo induction coils which, according to the prior art mentioned, are both directly effective upon the molten zone. Particularly difficult operating conditions clue to the levitating field have been encountered in cases where the molten zone passes downwardly through the rod be cause the control or regulating range available under such conditions is extremely small. It is particularly difficult in such cases to obtain recrystallizing material of uniform cross section since the levitating field presses the molten zone against the upper boundary face of the molten zone wmch for downward zone travel constitutes the recrystallization boundary. It is often infeasible to avoid this by reducing the levitating force as this force is preferably rated to just compensate but not over-compensate the effect of gravity upon the molten zone, so that a reduction in levitating force may cause the molten zone to drip off.

It is an object of my invention to improve methods and floating-zone melting apparatus of the above-mentioned general type toward affording a better control and regulation of the zone-heating and levitating effects. Another object, subsidiary to the one just mentioned, is to reliably provide for better uniformity in cross section of the zone-n1elted product with a greatly minimized tendency of incurring the above-mentioned trouble at the molten zone. Still another object of the invention is to afford separate controllability or regulation of the zonemelting and levitating functions respectively.

According to my invention, relating to the cruciblefree zone melting of vertically held rods of conducting, or semiconducting crystallizable material such as silicon, I produce or maintain the molten zone by high-frequency induction heating and I further subject the rod immediately above and below the molten zone to two auxiliary inductive fields for jointly providing a controlled levitating action upon the molten zone, the two auxiliary fields having the same frequency but mutually opposed instantane ous poling relative to their respective effects on the rod axis at a location in the molten zone.

According to another, more specific feature of my invention, the inductive heater coil for producing the molten zone and the two auxiliary inductance coils for controlled levitation are mounted together to form a single assembly relative to which the molten zone has a given location during performance of the zone melting process, so that all three inductance coils travel together with the molten zone relative to the longitudinal axis of the rod while the zone melting is in progress.

According to a further feature of the invention, the frequency of the alternating currents flowing through the auxiliary inductance coils above and below the molten zone are kept so low and the amplitudes of these respective currents are adjusted to such a high value that, on the one hand, the axial length of the molten zone is virtually determined only by the field of the heating coil and, on the other hand, the bulge due to gravity in the molten zone is displaced toward the middle height of the zone.

According to a further feature, it is preferable to pass the molten zone from above downward through the rod and to simultaneously take advantage of the resulting possibility that the contour angle at the upper freezing boundary of the molten zone, and consequently the cross section of the material recrystallizing out of the molten zone, can be adjusted by controlling or regulating the intensity of the current flowing through the upper one of the two auxiliary coils. It is also of advantage to give the power being transmitted from the inductive heating coil to the rod such a high rating that the axial length of the molten zone is considerably larger than, for example at least about twice as large as, the axial length of the inductive heater coil and the isothermal faces at the freezing boundaries of the molten zone are substantially planar. In this manner the semiconductor material crys-, tallizing out of the molten zone can be obtained in a condition of greatly minimized disclocations.

According to another feature of the invention, the inductive heater coil is energized by an alternating current of high amplitude and high frequency in comparison with the corresponding parameter values of the current in the auxiliary coils, so that the force which the field of the heater coil imposes upon the molten zone remains negligible in comparison with the resultant levitating action of the two other coils despite the high heating energy of the heater coil that determines the length of the molten zone.

The invention will be further explained with reference to the accompanying drawings in which:

FIG. 1 shows two inductance coils in operation rela tive to the molten zone of a rod as occurring with the prior methods mentioned above.

FIG. 2 is a comparative showing of three inductance coils acting upon a molten zone in accordance with the invention.

FIG. 3 shows schematically and by way of example a zone-melting device incorporating the coil assembly according to PEG. 2; and

FIG. 4 is a schematic circuit diagram of the same device.

While the invention is generally applicable to metals and semiconductors of any kind amenable to zone melting, it is preferably employed for the zone melting of semi conductor substances such as germanium and silicon. A particular advantage is afforded by the invention when employing it for pulling silicon monocrystals. In this case, it is preferable to pass the molten zone downwardly through the rod to avoid the danger, occurring when the zone travels in the upward direction, that small particles of material or impurities may drop upon the freezing monocrystalline boundary face. During downward travel this boundary face is on top so that the formation of a monocrystal is much more reliably secured. However, if a levitating field is provided by means of only one induction coil located beneath the heating coil, this type of operation results in the above-mentioned difficult operating conditions that afford an only small range of regulation. These conditions will be explained with reference to FIG. 1.

According to FIG. 1, the molten zone 2 is kept iloating between the two solid portions 2. and l of a silicon rod. The molten zone is produced or maintained by an inductive heater coil 3 and is supported by a levitating coil 4. The supporting field presses the molten zone upwardly and produces a relatively large, outwardly open contour angle. When the molten zone is passed from above downwardly through the rod, this contour angle has the effect of continuously enlarging the cross section of the material which reci'ystallizes out of the melt in monocrystalline constitution. The increase in monocrystallinc cross section is the larger the more the contour angle departs from 189. It is therefore desirable to maintain at the freezing boundary a tangential or contour angle as close as possible to 180. However, if one attempted to do this by reducing the levitating force caused by the lower coil l, the molten zone would descend already at slight changes in field strength of the lower coil i so that now the bulge of the zone would be near the lower boundary of the molten zone. This is likewise undesirable.

Both undesired phenomena can be virtually eliminated and an approximately 180 tangential angle be secured by the coil arrangement according to the invention exemplified by the embodiment shown in FIG/2.

This coil assembly comprises the inductive heater coil 3 for producing the molten zone 2 and the levitating coil 4 described above with reference to 1 1G; 1, but is provided with an additional auxiliary inductance coil 5 which presses the upper'portion of the molten zone 2 downwardly while imposing no appreciable force upon the lower portion of the molten zone. in this manner the bulge of the molten zone is displaced toward the middle, which is particularly favorable for uniform recrystallization.

FIG. 3 shows the same coil assembly as FIG. 2. The three coils 3, 4, 5 are rigidly mounted on a support 6 and consist preferably of copper tubing traversed by cooling water during operation. The cooling water is supplied to the support 6, for example, through flexible hose connections, and the electric currents for the respective coils 3, 4, 5 are supplied to the support 6 through flexible cables. The support 6 is shown engaged by a screw spindle 7 and the rod 1 is shown vertically mounted in holders 8 and 9. During operation, the spindle '7 is driven from a suitable 7 electric motor (not shown) for moving the support 6 with the coil assembly downward during zone melting and subsequently returning the assembly at faster speed upwardly during an idle return motion, whereafter another zone pass can be performed if desired. The illustrated device is preferably mounted within a processing vessel which may be filled with an inert gas or other protective atmosphere.

According to FIG. 4 the heater coil 3 is connected to a high frequency source ill, such as a suitable electronic generator having a frequency in the megacycle range, and supplying a current in the order of 100 amps, for example. The coils i and 5 are each connected to a radio-frequency source 12, for example an electronic generator having a frequency in the kilocycle range and a current rating higher than that of the generator ll. Shown in FIG. 4 are a control rheostat 13 for adjustingthe amplitude of the current flowing through the heater coil 3, and corresponding rheostats M and 15 for adjusting the amplitudes of the respective current passing through the auxiliary inductance coils l and 5.

The levitating forces at a constant amplitude of the magnetic field strength in an electromagnetic alternating field increase in proportion a lower power of the frequency than the heating power. it is therefore possible by employing f elds of relatively low frequency to secure the required levitating action it the amplitude of the field strength is made sufficiently large. Then the lower frequency does not cause a more intensive heating of the rod to be zone-inelted, despite the large field amplitude. Conversely, the required heating can be secured at low field amplitudes if the frequcncy of the heating alternating field is kept sufficiently high. For example, if the frequency of the alternating current flowing through the heater coil is made large while the field strength produced by this current at the location of the molten zone is keptsufiiciently small, such a field exerts only slight mechanical forces but has a great heating efiect upon the molten zone. Consequently the inductive heater coil, thus operated, cannot discernibly deform the molten zone, and the two other coils need develop only a correspondingly smaller force because they are not called upon to compensate for zone deformation due to the heating field. Conversely, the two auxiliary coils are traversed by respective currents of correspondingly high amplitude and low frequency, the current amplitude being so chosen that it is capable, despite the low frequency, of exerting the required force action upon the molten zone. The choice of a correspondingly low frequency counteracts the otherwise expectable increase in heat supply to the molten zone.

While the field of the lower auxiliary coil presses the molten zone upwardly in opposition to gravity, the upper auxiliary coil takes care that the molten zone is not pressed against the upper boundary of the molten zone. The effects of the two coils are so adapted to each other that the bulge of the molten zone due to gravity pressure becomes located at approximately the middle height of the zone. Since thus the lower auxiliary coil supports the molten zone against gravity, whereas the upper coil acts upon the molten zone in the same direction as gravity by pressing the zone downwardly, the two coils must be so dimensioned or be traversed by currents so rated that the force produced by the lower auxiliary coil has a considerably stronger axial component than the field of the upper coil because the lower coil must compensate gravity as well as a portion of the force effect due to the upper coil.

The particular frequency and current values can be chosen and adjusted in accordance with the requirements of the particular material and the dimensions of the particular rod to be processed. The following example is illustrative, relating to the zone melting of silicon. The rod diameter in this example was 18 min, the height of the molten zone 22 mm., its temperature about 1420 C. The heating coil 3 consisted of two turns located vertically above one another as apparent in Pl-G. 2, in contrast to the single-plane spiral turns of the auxiliary coils 4 and 5. The heater coil 3 was traversed by a current of A. and 4- mc. The spacing of the in er edge of heater coil 3 from the rod axis was 17 mm. The axial length of the heater winding was about 8 mm. The molten zone 2, therefore, protruded considerably upwardly and downwardly beyond the heater coil. The lower auxiliar coil 4 surrounded the rod about 13 mm. beneath the lower boundary of the melting zone. The inner edge of the lower coil 4 was likewise spaced 17 mm. from the rod axis. The coil l, constituting the main levitating coil' and consisting of six turns in a single plane, had an outer diameter of 47 mm. and was traversed by current of 300 A. and 10 kc. The counter coil 5 was located approximately 8 mm. above the lower boundary of zone 2 and was energized by a 10 lrc. current of 200 A. The upper coil 4, consisting of three spiral turns, thus produced a considerably weaker field than the lower coil 4. With the above-mentioned exemplified data, a troublefree conversion of polycrystalline silicon to monocrystalline silicon was obtained in a condition extremely poor in dislocations and having a uniform cross section of 18 the melting 5 mm. in diameter along the entire processed length of the rod.

As mentioned, all coils are perferably made of copper tubing and are traversed by liquid coolant during operation.

Upon a study of this disclosure, it will be obvious to those skilled in the art that my invention permits of a variety of modifications with respect to equipment, materials and parameter values and hence can be given embodiments other than particularly illustrated and described herein, without departing from the essential features of my invention and within the scope of the claim annexed hereto.

I claim:

The method for floating-zone melting a silicon rod, which comprises supporting the molten zone in a vertically held silicon rod by two induction coils coaxially surrounding the rod above and below the molten zone, energizing the two coils by alternating currents of the same frequency to respectively produce downwardly and upwardly acting fields on the rod axis at the location of References Cited by the Examiner UNITED STATES PATENTS 2,686,864 8/54 Wroughton 75--10 2,686,865 8/54 Kelly 7510 2,897,329 7/59 Matare 7510 2,904,411 9/59 Pfann 148--1.6 2,905,798 9/59 Freutel 1481.6

BENJAMIN HENKIN, Primary Examiner.

20 WINSTON A. DOUGLAS, DAVID L. RECK,

Examiners. 

